1. Field of the Invention
The present invention relates to semiconductor devices and a fabrication method thereof and relates in particular to MIS (Metal Insulator Semiconductors) type transistors and a fabrication method thereof.
2. Discussion of Background
Progress is being made in reducing the size of silicon devices for LSI (large scale integration) and rapid progress is being made in developing high performance devices with higher levels of integration according to scaling law. Along with making each element in the device smaller, scaling also lowers the supply voltage so that a relatively uniform electrical charge is applied to each element and allowing higher device performance using smaller elements. Currently, MOS-FET (Metal Oxide Semiconductor Field Effect Transistors) have a gate dielectric with a physical thickness of two nanometers or less. Making this gate dielectric even thinner is difficult due to the flow of large tunnel leakage current. The present gate dielectric size makes it impossible to increase the electrical charge in the inversion layer and hampers efforts to make higher performance devices.
To allow MOS-FET devices to handle larger current, the method of improving carrier mobility was considered. One method proposed to achieve better carrier mobility was a MOS-FET strained silicon channel made by epitaxial growth of a silicon layer on a layer of silicon-germanium. This method was for example disclosed in VLSI Tech. pp. 59–60 (2001) (hereafter referred to as Related Art 1). The strained silicon channel MOS-FET of this Related Art 1 utilized the difference in lattice spacing between the silicon-germanium layer and silicon layer to apply tensile stress to the silicon layer forming the device channel. Subjecting the silicon layer to tensile stress altered its band structure, giving it higher N and P type carrier mobility so that devices with a higher drive current could be obtained compared to silicon MOS-FET devices of the related art.
In order to develop a practical LSI strained silicon channel MOS-FET having this kind of high drive current, the preferred method used in recent years, forms a shallow trench isolation in contact with the silicon-germanium. However, the liner oxidation process which is essential in the related art for forming the shallow trench isolation is difficult to perform on silicon-germanium. The difficulty is due to the oxidized germanium being physically unstable compared to the oxidized silicon as well as to its poor isolation characteristics.
To resolve this problem, a method was proposed to epitaxially form a silicon layer on the surface of the silicon-germanium layer and then oxidize the silicon layer surface to form a shallow trench isolation. A method for example was disclosed in U.S. Pat. No. 5,266,813 (hereafter referred to as Related Art 2). This Related Art 2 achieves a shallow trench isolation with low leakage current by covering the inside of the shallow trench with an oxidized layer on the silicon layer. Further, since the silicon germanium layer is covered by a layer of silicon, no unstable oxidized germanium is formed during the oxidation process and no impurities occur from the oxidation furnace due to the germanium.
To utilize the shallow trench isolation in a MOS-FET, it is essential to control the crystal conformation and shape of the active region contacting the shallow trench isolated so it will not generate leakage current referred to as MOS-FET kink. However regulating the current leakage in this section is difficult with the technology of the related art. In the method of related art 2 for example, when developing epitaxial growth on the silicon layer on the surface of the silicon germanium layer, a polycrystalline silicon layer is simultaneously formed on the surface of the dielectric (insulator) remaining from the silicon nitriding and silicon oxidizing processes. This polycrystalline silicon on the edge of the active region is therefore a cause of leakage current, causing defects typified by increased standby current in the MOS-FET.